Gas turbine engines typically include a compressor for compressing a working fluid, such as air. The compressed air is injected into a combustor which heats the fluid causing it to expand, and the expanded fluid is forced through a turbine or turbines. The compressor may include a low pressure compressor and a high pressure compressor.
In some engine applications, such as on oil platforms, a heavy penalty is paid for fuel burned because of emissions taxes. Therefore, in such applications, it is desirable to provide increased power output at a same fuel flow (i.e., decreased specific fuel consumption) as is currently used. For such applications, the engine also preferably is low in weight and has a small volume compared to other power plant installations.
One known cycle which provides increased power output as compared to a direct open cycle is commonly referred to as an inverted Brayton cycle. In an inverted Brayton cycle, the high temperature exhaust gases from a main engine are expanded from high temperature and approximately atmospheric pressure to sub-atmospheric pressure. The air is then cooled at approximately constant, low pressure, and is recompressed to atmospheric pressure and discharged.
With an inverted Brayton cycle and due to the low pressures, the engine components, and particularly the turbine and compressor, must be much larger than those of a gas turbine working on a direct open cycle with high-pressure combustion. Particularly, additional power can be generated with an inverted Brayton cycle. Further, although an inverted Brayton cycle generates higher powers with any given compressor-turbine combination, its thermal efficiency is lower than in a high temperature direct open cycle. Specifically, the inverted cycle requires a cooler, which is not necessary in the direct cycle. The cooler adds a pressure drop in the cycle and takes heat away from the cycle, which adversely affects its thermal efficiency.
It would be desirable to provide a gas turbine engine which provides increased power output at a same fuel flow as is currently used in some known applications. It also would be desirable to provide such an engine which is low in weight and has a small volume compared to other power plant installations.